Light emitting diodes (LEDs) are solid-state light sources that work on the principle of the recombination of electrons and holes at the junction between a p-semiconductor and an n-semiconductor. The light emissions from LEDs are controlled by using different layers of luminescent materials deposited on the LED chip (substrate). The emissions of each layer are generally monochromatic. Different colors are achieved through the use of multilayers of luminescent materials and dyes. For example, multilayers of luminescent materials in the InGaAlP family grown onto suitable substrates can emit red, yellow or orange light. Multilayers of luminescent materials in the InAlGaN family grown onto SiC and Al2O3 substrates can emit blue, green or UV light.
To yield white light, output of three LEDs, e.g. a red, a green and a blue LED, can be combined. Alternatively, a single blue or UV LED can be used to excite a phosphor material that is placed in close proximity to the blue or UV LED. The phosphor material absorbs the blue or UV light and re-emits the light in a spectrum of including longer wavelengths. Thus, a phosphor coated blue LED can emit a spectrum of appropriate colors, which combine to produce white light.
White LEDs are produced by growing different layers of materials in the InAlGaN family using different doping substances to obtain the p-layers and n-layers. Organometallic vapor-phase epitaxy (OMVPE) is a common technique for growing such layers. In the OMVPE technique, organometallic molecules that contain the desired metallic atoms are transported in the gas/vapor phase onto a suitable substrate to yield a film on the substrate.
Examples of suitable substrates are Gallium Nitride (GaN), Aluminum Nitride (AlN), Aluminum oxide (Al2O3) and Silicon Carbide (SiC). The aluminum oxide and the silicon carbide may have a GaN or a AlN buffer layer between the substrate and the layers of light emitting diode structure. For example, the wafer substrate is covered completely with the layers of the light emitting diode structure which is then cut to create 10,000 LED dies per wafer. Each die is then mounted between two electrodes, to become the active element of the LED.
Phosphor material in the form of particles or thin film is deposited over the InAlGaN multilayer in order to shift the primary emission wavelength of the light emitting LED to the desired visible color emission spectrum. The phosphors comprise a host material such as YAG, CdS, ZnS, etc., that incorporate small concentrations of activator ions such as rare earth metals and transition metals. A description of phosphors is found in U.S. Pat. No. 6,466,135B1, which is incorporated herein by reference in its entirety. The InAlGaN multilayer LED with phosphors is typically encapsulated in a polymeric resin such as an epoxy resin. The encapsulation of LEDs in polymeric resins is described in U.S. Pat. No. 5,959,316, which is incorporated herein by reference in its entirety.
To increase the forward light emission of the LED, the phosphor covered multilayer LED can be placed in a suitable reflector cup. The reflector cup reflects the light emission towards the end of the LED.
The heat and UV energy from the LED can cause the polymeric resin encapsulation to degrade. The degradation of the polymeric resin encapsulation, in turn, causes a yellowing appearance of the light emission. Further, the emission efficiency of both the phosphor layer and the InAlGaN multilayer structure degrades in the presence of moisture. The oxidation state of the phosphor activator can change in the presence of oxygen, thus causing a reduction in light emission and a possible shift in emission wavelength. Since high power LEDs (e.g. white light LEDs) operate at significantly elevated temperatures, this oxidation reaction is temperature-enhanced. The change of color and intensity of the emission of LEDs is typically undesirable and especially so with white light LEDs.
FIG. 1 is a cross-sectional view of a prior art LED assembly 100 which includes a base or base 112. Of course, this assembly is just exemplary of a variety of types of LED assemblies. A paper describing several types of LED assemblies is “Packaging Challenges of High-Power LEDs for Solid State Lighting”, by Shatil Haque et al, Lumileds Lighting, San Jose, Calif., incorporated herein by reference.
In FIG. 1, an LED semiconductor material 106 is secured to a lead 108a. This can be accomplished using a silver-loaded conductive epoxy which provides high reflectivity. Alternatively, for “flip-chip” type LED structures solder-bump bonding can be advantageously used, as they do not hinder the extraction of light radiating from the active region.
Secured to the base 112 is a reflector cup 104a and leads 108a and 108b. Reflector cup 104a can be a solid mass of material having, for example, an inverted, truncated conical aperture providing a reflective surface 104b. The reflector cup 104a can be made from an electrically insulating material such as glass, ceramic or plastic. The reflective surface 104b can, for example, be a thin film of aluminum applied, for example, by a sputtering process. The leads 108a and 108b are typically made from a copper alloy. A bonding wire 107 electrically couples lead 108b to the top of LED material 106. A polymeric resin 103 is disposed within the conical aperture of the reflector cup 104a and encapsulates the LED material 106 and bonding wire 107. The encapsulant 103 preferably has interspersed, within it, phosphor materials 102 (shown as small bubbles) for shifting the emission wavelengths of the LED material to, for example, produce a “white” light. By “white”, it is meant herein that a broad spectrum of visible light is produced which produces a light which appears substantially white, although it may be somewhat tinged with certain frequencies (e.g. blue). A solid polymeric resin encapsulation dome 110 is bonded to the reflector cup 104a and over the encapsulant 103 such as by being epoxied or glued.
LED material 106 may include an InAlGaN multilayer LED structure. Base 112 may be made of an insulating material such as sapphire (Al2O3) or silicon carbide, for example. The base 112 should be very poor electrical conductor (e.g. an insulator), but is preferably a reasonably good thermal conductor. A heat sink (not shown) can be optionally attached to or form part of the base 112 to help dissipate heat from the LED assembly 100. Leads 108a and 108b are electrode leads and can be made of electrical conducting material, including copper and copper alloys. Leads 108a and 108b may be about the same width as the base of LED material 106. Reflector cup 104a may be made of an insulating material such as glass or ceramic, for example.
FIG. 2 illustrates certain problems encountered in the prior art LED assembly after a period of usage. After a period of time of operation of the LED 100, the ultraviolet (UV) light and heat energy generated by LED material 106 will cause the polymeric resin encapsulation dome 110 to discolor or “yellow.” This is illustrated by the discoloration 113 of the dome 110. The yellowing of the polymeric resin encapsulation dome 110, in turn, causes an absorption of light emitted from the InAlGaN multilayer structure and phosphor materials, particularly in the blue portion of the visible spectrum. This causes a reduction in light output from the LED, and a shift in the output color. Therefore, output of the LED 100 becomes less bright and less white (e.g. more yellow).
Further, both the phosphor materials and the InAlGaN multilayer structure react with moisture 115 (e.g. water vapor) that may diffuse through the solid polymeric dome and cause a degradation of the light emission intensity of the LED. Further, the moisture in assembly 100 can cause corrosion of the electrodes and other parts in assembly 100. The phosphor materials can also react with the oxygen 114 that may diffuse through the solid polymeric dome and cause a shift in the wavelength of the light emission. Thus, the effects of moisture and oxygen in the LED assembly are deleterious to the performance of the LED.
Based on the foregoing, there is a need for producing LED assemblies that maintain a high intensity and stable wavelength in the LED light emissions during operation, and which do not quickly degrade with use due to exposure to oxygen, moisture and/or other contaminants.